• Andy Davis

A polite, open letter to my monarch scientist colleagues regarding the population status


Greetings blog readers,

This week in monarch news, an open letter was just written and published in the scientific journal, the Annals of the Entomological Society of America, regarding the status of the eastern monarch population. These 'editorial letters' are fairly common in science, and are a way we scientists debate things, in a more or less public manner. They get published in the same way as regular scientific studies, although they are generally not 'peer-reviewed.' This letter was written by my colleagues who study monarchs, and most of whom I know very well, including John Pleasants, Lincoln Brower, Ernest Williams, Chip Taylor and Karen Oberhauser. They wrote it as sort of a critique of the introduction I wrote to the monarch special issue that came out last fall in the same journal (link here to go to the monarch collection). In leu of publishing a rebuttal letter of my own, I'm going to use this edition of my blog to reply to the critique. This post is not intended to be derogatory or insulting, just critical, which goes with th territory of being a scientist.

The link here will take you to their published letter in the journal - LINK. You should probably read it before reading the rebuttal below. If you don't read it, here are the bullet points of it - the monarch collection contained 7 papers on the long-term status of the eastern population, 3 of them showed no decline over time, 3 showed recent declines (last 7 years), and my introduction to the collection (link here) stated that the evidence for declines depends on which stage you look at (spring, summer, fall, etc.). My colleagues' letter took issue with this statement. So here goes my rebuttal...

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Dear readers and monarch enthusiasts,

One of the core elements of science, and by extension, its scientists, is that it is (and they are) objective. The nature of scientific objectivity means that when new data come to light that run counter to existing paradigms, scientists should adjust their thinking on the subject to match the new findings. The situation regarding conservation of monarch butterflies is a perfect example of the need for objectivity, and also on how this has been lost over time. For many years, monarchs in eastern North America has been gauged solely on the status of their overwintering colony sizes, which have shown clear declines in the past 2 decades (Brower et al. 2012b). This has led to the widespread narrative that ‘monarchs are declining’. In the recent publication in the Annals of seven articles devoted to long-term trends in monarch butterflies in eastern North America (Davis and Dyer 2015), several new data sets were analyzed that show no long-term declines during periods outside of the wintering season. Rather than reinterpret their stance on the population trajectory, Pleasants et al. have provided lengthy explanations for why the data from each of these three studies should not be used to gauge monarch populations. While I respect my colleagues who study monarchs, I would politely point out to them that their response is not consistent with the objective nature of science.

Specifically, there were three papers in the collection that showed little to no evidence for long-term declines or population contraction, including analyses of spring recolonization data (Howard and Davis 2015), counts of adults in the summer (Ries et al. 2015) and censuses of migrating adults in Michigan (Badgett and Davis 2015). In their letter to the editor, Pleasants et al. went to great lengths to point out flaws in each of these datasets that, in their opinion, make them inappropriate for estimating patterns of monarch abundance. While their arguments may or may not hold merit (I will not argue this here), I question why they are expending so much effort to dismiss these studies. After all, if one tries hard enough, one can find fault with nearly any scientific study, which is essentially what Pleasants et al. have done. I would also point out that each of these three articles was subjected to a standard peer-review process, where potential flaws in the data and/or methodology would have been identified and resolved.

The dismissive pattern running through their letter is reminiscent of a previous public debate I had with most of these same researchers on the same topic (Brower et al. 2012a, Davis 2012). There, I pointed out that there are two existing datasets from migratory censuses (at Cape May, NJ, and Peninsula Point, MI), that show no long-term decline, but their response then was that those sites are not ideally-located to show meaningful patterns. In the current debate, we now have two additional datasets on adult monarch abundance in the core summer breeding range (NABA and the Illinois Butterfly Monitoring Network) which also do not show declines (Ries et al. 2015), but my colleagues argue these data are not appropriate either because the census sites are not impacted by agricultural habitat losses or that the timing of the data collection was not acceptable (to them). Moreover, they also do not accept data derived from monarch spring recolonization that shows no long-term restrictions in breeding range size (Howard and Davis 2015), because of multiple perceived issues.

So by my count, collectively there are now a total of 5 long-term datasets of eastern monarchs that show no long-term declines in either the summer breeding range or the fall migration, and my colleagues have found various reasons to dismiss all of them. As a scientist, I find this to be troubling.

Here's the thing - if one accepts that the conclusions from the three monarch studies in question are valid, it means accepting that there is no decline in monarch abundance in the breeding season (and no one wants to do this). However we all know there is a clear decline in the size of the overwintering cohort in Mexico (no one disputes that). Therefore, a logical interpretation is that there are problems occurring during the migration (i.e. in the journey to Mexico), and I favor this idea. This interpretation means the migration itself should be the highest priority for conservation, which was stated in the introduction to the special issue, and which is a statement that Pleasants et al. clearly took issue with. Moreover, I would also argue this interpretation is the better path forward because it seeks to reconcile the differing patterns across datasets, rather than dismiss those that do not show declines, as Pleasants and colleagues have done.

Finally, I point out that the original intent of the special collection of monarch papers was to improve our understanding of monarch population dynamics outside of the overwintering phase, and to reduce reliance on the single dataset from Mexico. Unfortunately we are now faced with the situation where we have multiple graphs from throughout the monarch life cycle, but they show conflicting long-term patterns. How then do we characterize the population trajectory now? Given that we are having this debate (my response to their rebuttal letter), on top of the prior debate over the same topic (Brower et al. 2012a, Brower et al. 2012b, Davis 2012), and that there is clear disagreement among the many datasets we have on monarchs, one could argue that the most objective way to describe the long-term trend in monarch butterflies is that it is ‘debatable’, or ‘unclear for now’. To continue to argue that monarchs are declining despite the conflicting evidence is unjustified, and to simply find reasons to dismiss datasets that do not show declines is not objective.

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OK, now that I've posted this letter and made these points, let me finish this blog post by saying everything here is moot anyway - it's pretty clear that the public believes monarchs are declining, there are already millions of dollars flowing into monarch conservation based on this perception, and monarchs are even being considered for federal protection. What does it matter then if a monarch scientist or two does not believe it?

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Literature Cited (bold papers are from the monarch collection):

Badgett, G., and A. K. Davis. 2015. Population trends of monarchs at a northern monitoring site: analyses of 19 years of fall migration counts at Peninsula Point, MI. Annals of the Entomological Society of America 108: 700-706.

Brower, L. P., O. R. Taylor, and E. H. Williams. 2012a. Response to Davis: choosing relevant evidence to assess monarch population trends. Insect Conservation and Diversity 5: 327-329.

Brower, L. P., O. R. Taylor, E. H. Williams, D. A. Slayback, R. R. Zubieta, and M. I. Ramirez. 2012b. Decline of monarch butterflies overwintering in Mexico: is the migratory phenomenon at risk? Insect Conservation and Diversity 5: 95-100.

Davis, A. K. 2012. Are migratory monarchs really declining in eastern North America? Examining evidence from two fall census programs. Insect Conservation and Diversity 5: 101-105.

Davis, A. K., and L. A. Dyer. 2015. Long-term trends in eastern north american monarch butterflies: a collection of studies focusing on spring, summer, and fall dynamics. Annals of the Entomological Society of America 108: 661-663.

Howard, E., and A. K. Davis. 2015. Investigating long-term changes in the spring migration of monarch butterflies (Lepidoptera: Nymphalidae) using 18 years of data from Journey North, a citizen science program. Annals of the Entomological Society of America 108: 664-669.

Pleasants, J. M., E. H. Williams, L. P. Brower, K. S. Oberhauser, and O. R. Taylor. 2015. Letter to the Editor: Conclusion of no decline in summer monarch population not supported. Annals of the Entomological Society of America. Online early. http://dx.doi.org/10.1093/aesa/sav115.

Ries, L., D. J. Taron, and E. Rendon-Salinas. 2015. The disconnect between summer and winter monarch trends for the eastern migratory population: possible links to differing drivers. Annals of the Entomological Society of America 108: 691-699.

Stenoien, C., K. R. Nail, and K. S. Oberhauser. 2015. Habitat productivity and temporal patterns of monarch butterfly egg densities in the eastern United States. Annals of the Entomological Society of America 108: 670-679.

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